A medical device (MD) can be implanted in a patient for various different purposes, including, but not limited to, treating physiologic conditions, monitoring physiological conditions, treating neurological conditions, monitoring neurological conditions, diagnosing diseases, treating diseases, or restoring functions of organs or tissues. Where the MD is implanted, it is often referred to as an implantable medical device (IMD). Examples of IMDs include, but are not limited to, implantable neurostimulators, implantable cardiac rhythm management devices (e.g., implantable cardioverter defibrillators and pacemakers) and drug delivery devices. Because such a device may be implanted in a patient for a long time, the size and power consumption of the device are inherently constrained. For this and other reasons, an IMD may depend on an external (i.e., non-implanted) system, generically referred to as a base station (BS), to perform certain functions. Such a non-implanted BS can be a patient programmer, a clinical programmer or a remote monitoring device, but is not limited thereto.
An implantable neurostimulator (INS) is an IMD that performs neurostimulation, which has become an accepted treatment for patients with chronic pain in their back and/or limbs who have not found pain relief from other treatments. In general, neurostimulation involves applying an electrical current to nerve tissue in the pathway of the chronic pain. This creates a sensation that blocks the brain's ability to sense the previously perceived pain. There are two conventional forms of electrical stimulation commonly used to treat chronic pain: Spinal Cord Stimulation (SCS) and Peripheral Nerve Field Stimulation (PNFS). In SCS, electrical leads are placed along the spinal cord. A programmable INS is typically implanted in the upper buttock or abdomen (under the skin) and emits electrical currents to the spinal cord via electrodes of the leads. Peripheral nerve field stimulation is similar to spinal cord stimulation, however peripheral nerve field stimulation involves placing the leads just under the skin in an area near to the peripheral nerves involved in pain.
Wireless communication between an IMD and an external BS is often referred to as telemetry. Examples of specific telemetry functions include, but are not limited to, programming or instructing the IMD to perform certain therapeutic tasks and/or adjust certain therapeutic parameters, downloading firmware upgrades to the IMD, uploading operational status information (e.g., battery and/or impedance measurements) from the IMD, and uploading data stored within the IMD. A useful type of wireless communication is radio frequency (RF) communication since it does not require that the BS and the IMD be very close to one another. Rather, with RF communication the BS and the IMD can be many feet apart while still allowing for reliable communication.
A non-implanted BS and an IMD, such as an INS, can communicate using the Medical Implant Communication Service (MICS) standard, which was defined by the U.S. Federal Communications Commission (FCC) and European Telecommunications Standards Institute (ETSI). The MICS standard uses the RF band between 402 and 405 MHz to provide for bi-directional radio communication with IMDs, such as an INS. The RF band between 402 and 405 MHz can be broken down into multiple channels, e.g., into ten 300 kHz wide channels, but not limited thereto. In 2009 the FCC began referring to the RF band between 402 and 405 MHz as being part of the 401 to 406 MHz Medical Device Radiocommunications (MedRadio) Service band. Accordingly, for the remainder of this description, the RF band between 402 and 405 MHz will be referred to as the MICS/MedRadio band, and the communication standards relating to the MICS/MedRadio band will be referred to as the MICS/MedRadio communication standards.
A non-implanted BS can be powered by a battery or can be plugged into an AC power socket. Even if powered by a battery, the battery of the BS can be relatively easily recharged and/or replaced. By contrast, an IMD, such as an INS, typically must be powered by a battery that is difficult and/or invasive to replace and/or recharge. Additionally, there are size limitations for the battery of an IMD. Thus, it is important for an IMD to conserve as much battery power as possible.
The battery of an IMD, such as an INS, is typically used to provide the energy for delivering stimulation, as well as the energy used to provide wireless communication (also referred to as telemetry) between the IMD and a non-implanted BS. Where RF communication is used, the IMD will typically include an RF transceiver, as will the non-implanted BS. The RF transceiver of the non-implanted BS will typically act as a master device and the RF transceiver of the IMD will typically act as a slave device during the RF communication, but that need not be the case. The RF transceiver of the IMD requires a relatively large amount of energy to maintain an RF link with the RF transceiver of the non-implanted BS. This power usage often increases as the battery voltage of the IMD becomes lower since a battery boost may need to be applied to provide the necessary voltage to the RF transceiver.